1. Technical Field
The invention relates to a method of controlling the nominal fill and purge times used in connection with an emission control device to facilitate xe2x80x9clean-burnxe2x80x9d operation of an internal combustion engine.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select constituent gases, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d The timing of each purge event must be controlled so that the device does not otherwise exceed its NOx storage capacity, because NOx would then pass through the device and effect an increase in tailpipe NOx emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
The prior art has recognized that the storage capacity of a given emission control device is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent gas may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-storing efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity.
The amount of the selected constituent gas that is actually stored in a given emission control device during vehicle operation depends on the concentration of the selected constituent gas in the engine feedgas, the exhaust flow rate, the ambient humidity, the device temperature, and other variables. Thus, both the device capacity and the actual quantity of the selected constituent gas stored in the device are complex functions of many variables.
When the engine is operated using a fuel containing sulfur, sulfur is stored in the device and causes a decrease in both the device""s absolute capacity to store the selected constituent gas, and the device""s instantaneous efficiency to store the selected constituent gas. When such device sulfation exceeds a critical level, the absorbed. SOx must be xe2x80x9cburned offxe2x80x9d or released during a regeneration or desulfation event, during which device temperatures are raised above perhaps about 650xc2x0 C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a device desulfation method which includes raising the device temperature to at least 650xc2x0 C. by introducing a source of secondary air into the exhaust upstream of the NOx device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of SOx.
It is an object of the invention to provide a method and system for controlling an emission control device based on the depletion of the device""s capacity to store a selected engine-generated constituent gas.
Under the invention, a method of filling and purging an emission control device, located in an exhaust passage of an engine upstream from an oxygen sensor, is provided so that the device is substantially filled to capacity with a constituent gas of the engine-generated exhaust gas during a fill time and is substantially emptied of previously-stored constituent gas during a subsequent purge time. The method includes calculating the depletion of device capacity of the device based on a calibrated device filling rate in engine speed load regions multiplied by the time spent in the region; continuously summing the calculated depletion; and scheduling a purge event when the summed calculated depletion exceeds a predetermined value for capacity depletion, wherein the predetermined value for capacity depletion is less than 100%.
In accordance with a feature of the invention, the calibrated device filling rate is preferably modified as a function of air-fuel ratio, EGR and spark advance and, further, as a function of device temperature. In an exemplary embodiment, the device filling rate is advantageously mapped over an engine speed and load operating range using a representative calibration for device temperature, air-fuel ratio, EGR and spark advance, with the depletion of device capacity being a time-weighted sum based on the amount of time spent in a given speed-load region utilizing the mapped filling rates in each region modified to account for the difference between existing and calibrated operating conditions.
In accordance with another feature of the invention, the method preferably further includes purging the device for a purge time tP(k), and monitoring the output signal of an oxygen sensor to determine the purge time tP(k+1) for the next purge cycle based on the output voltage of the sensor. In this manner, the purge time is iteratively optimized for the current storage capacity of the device. Most preferably, the fill time is likewise optimized by adjusting the fill time using predetermined increments that are respectively greater than and less than an initial fill time until the change in the purge time with respect to the corresponding fill time equals a predetermined target value.
According to yet another feature of the invention, the method further preferably includes scheduling a device regeneration event, such as a desulfation event, is the thus-adjusted purge time is less than a predetermined purge time for a non-deteriorated device minus an offset. Most preferably, the method includes scheduling at least one additional regeneration event if the device purge time does not increase as a result of a previous regeneration cycle; and, upon the scheduling of a predetermined number of additional regeneration cycles, triggering a warning indication.
From the foregoing, it will be appreciated that the invention beneficially utilizes information related to the quantity of a constituent gas of the engine-generated exhaust gas that is stored in an emission control device, along with the maximum capacity of the device to store such constituent gas, to adjust device purge parameters, storage capacity depletion rate (the rate at which device storage capacity is decreased during the filling process), purge time, and strength of purge in real time during engine operation in a vehicle. Still further, a decision to regenerate the device, for example, by purging the device of accumulated sulfur, is made based on the observed reduction in device storage capacity and the related increase in the storage capacity depletion rate. Thus, the device is operated continuously at its optimum condition of constituent-gas conversion efficiency, thereby minimizing tailpipe emissions while maximizing vehicle fuel economy. Intelligent regeneration of the device ensures that the constituent-gas conversion efficiency of the device is always maintained above a given minimum.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.